The present invention relates to a new process for the manufacture of monopotassium phosphate. More particularly, the present invention relates to a new process for the manufacture of monopotassium phosphate using the raw material KCl as a source of potassium.
The direct neutralization of phosphoric acid by potassium hydroxide or carbonate is readily seen to be uneconomic, if the product is to be used as a fertilizer and therefore most of the known methods are based on the utilization of potassium chloride as starting material.
The known methods for the manufacture of monopotassium phosphate, from phosphoric acid and potassium chloride, are based on the thermal reaction between the two reagents in which the following reaction occurs: EQU H.sub.3 PO.sub.4 +KCL.fwdarw.KH.sub.2 PO.sub.4 +HCl
However the yield of the said reaction is poor unless a large excess of phosphoric acid is used, in which case the solid product obtained is a double salt: KH.sub.2 PO.sub.4.H.sub.3 PO.sub.4 which requires a further step for its decomposition and recovery of the excess of phosphoric acid. A further drawback in the thermal process is the need for pure starting materials, as all impurities present in the feed potassium chloride and phosphoric acid are carried through the solid product. Lastly, the thermal process requires temperatures above 160 degrees C., which cause problems of corrosion and could lead to the formation of insoluble potassium metaphosphate, unless special precautions are taken such as that proposed in the U.S. Pat. No. 3,393,044 wherein organic substances are added to the reaction mixture so as to expel the hydrogen chloride at a lower temperature. However, even with that improvement the reaction temperature remains in the range of 120 to 160 degrees C., so that the corrosion problems remain.
Another approach for the manufacture of monopotassium phosphate at ambient temperature, was described in the U.S. Pat. No. 2,902,341 (Baniel et. al.), wherein potassium chloride is reacted with large excess of phosphoric acid and the mixture of hydrochloric acid (resulted in the reaction) and the excess of phosphoric acid are removed by solvent extraction using water immiscible organic solvents. The monopotassium phosphate is crystallized from the aqueous solution resulting after the removal of the acids. The implementation of this process on a commercial scale is questionable, particularly in view of the high costs involved concerning the organic solvents and energy required to separate phosphoric acid from the hydrochloric acid.
According to U.S. Pat. No. 3,661,513 a process is disclosed for the manufacture of monopotassium phosphate from phosphoric acid and potassium chloride, wherein the reaction mixture is treated with an immiscible secondary amine to selectively extract the chloride ion. The solution recovered has a reduced anion content and a pH of about 7. On evaporating the so-treated phosphate solution, solid monopotassium phosphate can be recovered. The advantage mentioned to exist in this method is that the product is substantially free of contaminating anions without the use of the high temperatures mentioned by the prior art.